The present invention relates generally to an electric double layer capacitor, and more particularly to a high performance double layer capacitor made with low-resistance aluminum-impregnated carbon-cloth electrodes and a high performance electrolytic solution.
Double layer capacitors, also referred to as electrochemical capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, they can typically deliver the stored energy at a higher power rating than rechargeable batteries. Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to an ionic) current from shorting the cell. Coupled to the back of each of the active electrodes is a current collecting plate. One purpose of the current collecting plate is to reduce ohmic losses in the double layer capacitor. If these current collecting plates are non-porous, they can also be used as part of the capacitor seal.
Double layer capacitors store electrostatic energy in a polarized liquid layer which forms when a potential exists between two electrodes immersed in an electrolyte. When the potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name "double layer" capacitor) by the polarization of the electrolyte ions due to charge separation under the applied electric field, and also due to the dipole orientation and alignment of electrolyte molecules over the entire surface of the electrodes.
The use of carbon electrodes in electrochemical capacitors with high power and energy density represents a significant advantage in this technology because carbon has a low density and carbon electrodes can be fabricated with very high surface areas. Fabrication of double layer capacitors with carbon electrodes has been known in the art for quite some time, as evidenced by U.S. Pat. No. 2,800,616 (Becker), and U.S. Pat. No. 3,648,126 (Boos et al.).
A major problem in many carbon electrode capacitors, including double layer capacitors, is that the performance of the capacitor is often limited because of the high internal resistance of the carbon electrodes. This high internal resistance may be due to several factors, including the high contact resistance of the internal carbon-carbon contacts, and the contact resistance of the electrodes with a current collector. This high resistance translates to large ohmic losses in the capacitor during the charging and discharge phases, which losses further adversely affect the characteristic RC (resistance.times.capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time. There is thus a need in the art for lowering the internal resistance, and hence the time constant, of double layer capacitors.
Various electrode fabrication techniques have been disclosed over recent years. For example, the Yoshida et al. patent (U.S. Pat. No. 5,150,283) discloses a method of connecting a carbon electrode to a current collector by depositing carbon powder and other electrical conductivity-improving agents on an aluminum substrate.
Another related approach for reducing the internal resistance of carbon electrodes is disclosed in U.S. Pat. No. 4,597,028 (Yoshida et al.) which teaches that the incorporation of metals such as aluminum into carbon fiber electrodes can be accomplished through weaving metallic fibers into carbon fiber preforms.
Yet another approach for reducing the resistance of a carbon electrode is taught in U.S. Pat. No. 4,562,511 (Nishino et al.) wherein the carbon fiber is dipped into an aqueous solution to form a layer of a conductive metal oxide, and preferably a transition metal oxide, in the pores of the carbon fibers. Nishino et al. also discloses the formation of metal oxides, such as tin oxide or indium oxide by vapor deposition.
Still another related approach for achieving low resistance is disclosed in U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963 (Tatarchuk et al.). The Tatarchuk et al. patents demonstrate that metal fibers can be intermixed with a carbon preform and sintered to create a structurally stable conductive matrix which may be used as an electrode. The Tatarchuk et al. patents also teach a process that reduces the electrical resistance in the electrode by reducing the number of carbon-carbon contacts through which current must flow to reach the metal conductor. This approach works well if stainless steel or nickel fibers are used as the metal. However, applicants have learned that this approach has not been successful when aluminum fibers are used because of the formation of aluminum carbide during the sintering or heating of the electrode.
Another area of concern in the fabrication of double layer capacitors relates to the method of connecting the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor, and such internal resistance must be kept as low as possible.
U.S. Pat. No. 4,562,511 (Nishino et al.) suggests plasma spraying of molten metals such as aluminum onto one side of a polarizable electrode to form a current collector layer on the surface of the electrode. Alternative techniques for bonding and/or forming the current collector are also considered in the '511 Nishino et al. patent, including arc-spraying, vacuum deposition, sputtering, non-electrolytic plating, and use of conductive paints.
The previously-cited Tatarchuk et al. patents (U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963) show the bonding of a metal foil current collector to the electrode by sinter bonding the metal foil to the electrode element.
U.S. Pat. No. 5,142,451 (Kurabayashi et al.) discloses a method of bonding the current collector to the surface of the electrode by a hot curing process which causes the material of the current collectors to enter the pores of the electrode elements.
Still other related art concerned with the method of fabricating and adhering current collector plates can be found in U.S. Pat. Nos. 5,065,286; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 all issued to Kurabayashi et al.
It is thus apparent that there is a continuing need for improved double layer capacitors. Such improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and energy density ratings Within a relatively short period of time. Such improved double layer capacitors should also have a relatively low internal resistance and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is also apparent that improvements are needed in the techniques and methods of fabricating double layer capacitor electrodes so as to lower the internal resistance of the double layer capacitor and maximize the operating voltage. Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. It is thus readily apparent that improved techniques and methods are needed to lower the internal resistance of the electrodes used within a double layer capacitor and increase the operating voltage.